1. Field of the Invention
The present invention relates to the field of electric drives. It refers to a control system for a drive having an asynchronous motor, in particular for a railroad vehicle, which control system comprises a first control loop for motor control and a second control loop, which is connected to the first control loop and has an adhesion controller.
Such a control system, in which the motor control is torque-based and the nominal torque for motor control is emitted by the higher-level adhesion controller on the basis of the rotation speed measured on the motor shaft, is known from the prior art.
2. Discussion of Background
In the case of electrically driven locomotives having torque-controlled drive motors in the form of asynchronous motors (ASM), wheel-slip situations can occur, in which the drive wheels spin to a greater or lesser extent in an uncontrolled manner. This results in unstable operation, in which the maximum possible traction between the wheel and rail is not achieved. At the same time, the drive wheels and the rails are subject to increased wear.
A large number of solution proposals as well as practically implemented methods exist to overcome the problem. One known torque-based control system is shown, by way of example, in FIGS. 1 to 4: the method is based on a cascade of two control loops, which can be seen in the schematic illustration in FIG. 1. The control system 100 comprises the process 104, the motor controller 103, the adhesion controller 102 and, possibly, an intermediate-circuit antiphase controller 101. For its part, according to FIG. 2, the process 104 comprises an invertor 108, which is connected on the input side via an intermediate-circuit capacitor 107 to a DC intermediate circuit 105 and is controlled via switching commands S.sub.R, S.sub.S and S.sub.T, and an asynchronous motor 113, which is supplied by the invertor 108 and, via a motor shaft 112, a gearbox 114 and a wheel 115, attempts to transmit power via appropriate friction to the rail 116. A rotation speed sensor 111 is fitted on the motor shaft 112, measures the rotation speed n.sub.shaft of the motor shaft 112, and emits this rotation speed n.sub.shaft to the control system for further processing. Other input variables required for the control system are two of the three phase currents i.sub.R and i.sub.S, which are tapped off from the supply cables via current sensors 109 and 110, as well as the intermediate-circuit voltage u.sub.d, which is measured by a voltage sensor 106 in the intermediate circuit 105.
The variables u.sub.d, i.sub.R,s and n.sub.shaft obtained from the process 104 are fed back to the motor controller 103. By comparison with a predetermined dynamic nominal torque value M.sub.s,nom, these variables are used in the motor controller 103 to derive the control commands S.sub.R,S,T for the invertor 108, and to pass said commands to the process 104. The derivation is carried out, for example, in the starting range in accordance with the block diagram (illustrated in FIG. 3) of the so-called indirect self-control system ISR, as is known from the document Elektrische Bahnen [Electric railroads] 89(1991), Issue 3, pages 79-87: a so-called motor monitor 117, that is to say a computation circuit which contains models of the invertor 108 and of the asynchronous motor, uses the said input variables to calculate an actual stator flux value Psi.sub.act and an actual torque value M.sub.act for the asynchronous motor 113.
A flux controller 118 uses the difference between a predetermined nominal stator flux value Psi.sub.nom and the actual stator flux value Psi.sub.act to derive a stator flux correction value k.sub.Psi. A torque controller 119 uses the difference between a predetermined dynamic nominal torque value M.sub.s,nom and the actual torque value M.sub.act to derive a dynamic nominal stator frequency f.sub.s,nom which, together with a steady-state nominal stator frequency f.sub.T,nom produces the nominal stator frequency f.sub.nom. The nominal stator frequency f.sub.T,nom is obtained via an initial controller 120 from the nominal torque value M.sub.s,nom and by superimposition of the rotation speed n.sub.shaft. A first calculation block 121 uses the input variables k.sub.Psi and f.sub.nom to calculate the change .DELTA.Psi in the stator flux vector, and a second calculation block 122 uses this to calculate the voltage vector u of the motor voltage, while a downstream pulse-width controller 123 uses this to derive the necessary switching commands S.sub.R,S,T for the invertor 108. In other rotation speed ranges, the variables are derived in a different manner, for example using the method of direct self-control DSR (once again, see the abovementioned document in this context).
The upper control loop described in FIG. 1 forms an inner control loop which allows highly dynamic control of the torque of the traction motor or motors. Superimposed on this is a second, lower control loop, which contains an adhesion controller 102. The adhesion controller 102 is intended to stabilize the drive in the case of varying friction conditions between the wheel 115 and the rail 116 and, if required, to attempt to find the traction maximum. For this purpose, it contains a traction force and slip controller as well as a device to search for the traction maximum. The interface between the two control loops is the dynamic nominal torque value M.sub.s,nom and the measured rotation speed n.sub.shaft of the motor shaft, or of the rotor of the traction motor. The dynamic nominal torque value M.sub.s,nom may in this case be composed of the steady-state nominal torque value M.sub.T,nom emitted by the adhesion controller 102 and an additional torque correction signal M.sub.s,ud from an intermediate-circuit connection, which is derived from the intermediate-circuit voltage u.sub.d by an additional intermediate-circuit antiphase controller 101 for damping oscillations in the DC intermediate circuit 105.
The superimposed second control loop, together with the adhesion controller 102, derives its information about the slip state from the rotation speed signal n.sub.shaft, and is therefore provided with good rotation speed detection. In the case of the known control system structure according to FIG. 1, it is thus disadvantageous that the control system generally fails at low traction speeds (low rotation speeds n.sub.shaft) since, on the one hand, the rotation speed information in this range is inadequate (number of pulses per revolution) and, on the other hand, actual rotation speed sensors 111 have non-ideal characteristics (eccentricity, signal noise resulting from pulse tolerances etc.). In practice, a very high, undesirable maintenance cost is required to overcome these problems.
Furthermore, in order to achieve low-wear operation, active damping of the drive mechanism is required, since the mechanical spring/mass system of the drive generally has only very weak damping. Owing to the said non-ideal characteristics of the rotation speed signal n.sub.shaft, this is often not feasible, or is feasible only to an inadequate extent.
Finally, control methods using an impressed torque--as is shown in FIG. 4--are not able to use the falling branch of the wheel slip characteristic or traction characteristic A (power F plotted against the difference between the wheel and rail speeds dv) to set a stable operating point, for example for a power level F' and a speed difference dv.sub.nom, since the increasing rotation speed is no longer counteracted by a higher load torque. Stabilization by means of a rotation speed controller is extremely time-critical, and the unavoidable dead time in the rotation speed measurement stimulates oscillations about the operating point (dashed lines in FIG. 4).